1. Field of the Invention.
This invention pertains to a novel process for fracturing subterranean formations and to novel aqueous fracturing fluids containing novel zirconium crosslinkers.
2. Description of the Prior Art.
Hydraulic fracturing is a term that has been applied to a variety of techniques used to stimulate the production of oil, gas and other formations fluids from subterranean formations. In hydraulic fracturing, a suitable fracturing fluid is introduced into a subterranean formation by way of a wellbore under conditions of flow rate and pressure which are at least sufficient to create and/or extend a fracture into the desired part of the formation. The fracturing fluid normally carries with it a proppant (e.g. sand, bauxite, etc.) which is forced into the fracture itself and keeps the broken formation from closing down upon itself once the pressure is released.
Various fluids under pressure have been used in hydraulic fracturing. Most of the fracturing fluids used today are aqueous-based liquids which have been either gelled or foamed.
Aqueous gels are usually prepared by blending a polymeric gelling agent with an aqueous medium. Most frequently, the polymeric gelling agent of choice is a solvatable polysaccharide. These solvatable polysaccharides form a known class of compounds which include a variety of natural gums as well as certain cellulosic derivatives which have been rendered hydratable by virtue of hydrophilic substituents chemically attached to the cellulose backbone. The solvatable polysaccharides therefore include galactomannan gums, glucomannan gums, cellulose derivatives, and the like.
The solvatable polysaccharides have a remarkable capacity to thicken aqueous liquids. Even small amounts are sufficient to increase the viscosity of such aqueous liquids from 10 to 100 times or more. In many instances, the thickened aqueous liquid has sufficient viscosity to carry the proppant during the course of the fracturing process and represents a satisfactory fracturing fluid. In other instances, it is necessary to crosslink the polysaccharide in order to form a gel having sufficient strength and viscosity to carry the proppant. A variety of crosslinkers have been developed to achieve this result.
The borate ion has been used extensively as a crosslinking agent for hydrated guar gums and other galactomannans to form aqueous gels used in fracturing and other areas. For example, Kern described a crosslinked system in U.S. Pat. No. 3,058,909 which was used extensively in the oil and gas industry as a fracturing fluid. A fracturing process which comprised crosslinking, guarcontaining compositions on-the-fly with borate ions was described by Free in U.S. Pat. No. 3,974,077. The borate-crosslinked systems require a basic pH (e.g. 8.5 to 10) for crosslinking to occur.
Other crosslinking agents were developed using certain transition metals. Chrisp described certain of these crosslinked systems in U.S. Pat. Nos. 3,202,556 and 3,301,723. In U.S. Pat. No. 3,202,556, aqueous solutions of galactomannan gums were crosslinked at a pH of from about 6 to 13 with crosslinking agents selected from the group consisting of compounds of antimony and bismuth. In U.S. Pat. No. 3,301,723 Chrisp described the use of certain titanium, zirconium, and other transition metals as crosslinking agents for galactomannan gums at a pH also in the range from about 6 to about 13. In both Chrisp patents, a basic pH was used to prepare crosslinked materials having utility in the explosive industry.
Another patent which described the use of titanium crosslinkers for solvatable polysaccharides was Tiner et al. (U.S. Pat. No. 3,888,312). The crosslinked gels formed by Tiner were said to be useful as fracturing fluids. The use of such crosslinked gels was alleged to overcome the high friction loss experienced during the pumping of many high viscosity fracturing fluids previously known. This observation corroborated the disclosure by Chrisp in U.S. Pat. No. 3,301,723 at column 10 that crosslinked gels formed using titanium, chromium, iron, and zirconium crosslinkers had a high surface tension (i.e. stickiness and tackiness are absent), ready workability and other desirable physical characteristics.
Chrisp and Tiner et al. each described titanium crosslinkers in which the "amine" portion of the crosslinker was a residue of triethanolamine. Chrisp in U.S. Pat. No. 3,301,723 at column 5, line 60 identified the crosslinker as titanium-triethanolamine chelates. Tiner et al. in U.S. Pat. No. 3,888,312, column 3 at lines 32-35 identifies the compound as bis(triethanolamine) bis(isopropyl)titanium (IV). Chrisp and Tiner et al. also disclosed a wide variety of other compounds in which the "anion" portion of the molecule was something quite different than the triethanolamine residue (e.g. chloride). Chrisp in U.S. Pat. No. 3,301,723 at column 4, lines 33-39 taught that the nature of the particular "anion" in the crosslinking agent was not critical but did have an influence on the solubility of the crosslinking compounds. Tiner et al. likewise taught that the crosslinking ability of their titanium crosslinking agents depended upon the presence of titanium in the +4 oxidation state and that the "anion" portion of the molecule could be varied. This broad teaching in Tiner et al. is once again analogous to the teaching in Chrisp.
Reference is made to the "Handbook of WaterSoluble Gums and Resins" by Robert L. Davidson, Editor as published by McGraw-Hill, Inc. (1980) for an excellent treatise on water soluble polymers which includes a discussion on hydratable (or solvatable) polysaccharides. Reference is also made to "Hydraulic Fracturing" by G. C. Howard and C. R. Fast, Monograph Volume 2, Henry L. Doherty Series, published by the Society of Petroleum Engineers (1970) which is an excellent introduction to the subject of hydraulic fracturing, even though it is now somewhat dated.